Telephone set



Nov. 10,1925 1,560,768

A. S. CURTIS TELEPHONE SET Filed Dec. 24, 1920 F/q/ F/qZ 6' f i i T i Patented Nov. 10, 1925.

UNITED STATES PATENT OFFICE.

ALFRED S. CURTIS, OI' NRW YORK, N. ,Y., ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK,'N. Y., A CORPORATION OF NEW YORK.

' TELEPHONE SET.

Application filed December 24, 1820. Serial No. 438,008. 7

To all whom. it. may concern: Be it known that I, -ALFRED S. CURTIS, a citizen of the United States of America, residing at New York, in the countyof New 6 York and State of New'York, have invented certain new and useful Improvementsin- Telephone Sets, of which the following is a full, clear, concise, and exact description.

This invention relates to telephone sets in 10 which the transmitter and receiver are mounted on a common support and particularly it relates to telephone hand sets.

It has long. been known that when the receiver and transmitter of a telephone set are connected together upon a commonhandle, a noise often referred to as howling, is experienced when "the instrumentis placed in operation. This effect has heretofore prevented the use of highly efficient-receivers and transmitters since they, by virtue "of their higher efficiency, limit the conditions under which the sets maybe used without howling. Thereware, atthe present time, many t pes of hand sets in use, but investigation as shown that the efiiciency of the transmitters and receivers used in connection therewith is necessarily considerably less than that of the instruments employed in the better type of desk or wall telephones.

It is the purpose of this invention to-provide a hand set and the like which may be used with highl eflicient transmitters and receivers and w ich will not tend to howl under the usual conditions of operation.

It has been found that the howling which takes lace in telephone hand sets is governed y the relation of the uni-directional mutual impedance between-the receiver and the transmitter to the transfer impedance between them through the-electrical circuit. The uni-directional mutual impedance, so called because it exists in one direction only, is defined as the ratio of the voltage generated in the transmitter, due to mechanical.

and accoustical vibrations communicated to- 1t by the receiver, to the receiver current by it in the receiver. When the uni-direc-.

tional mutual impedance will result in" a corresponding decrease in the howling.

The variable factors which affect the ultimate value of the uni-directional mutual impedance are: 1

Impedance of the receiver,- Impedance of the transmitter, Efiiciencyof the receiver,

Efliciency of the transmitter, v Mechanical construction of the set. "Heretofore no great attention has been paid to these'several factorspwith the result that only receivers and transmitters of coin paratively' low efliciency co uld' be-"used in order to insure absence of liowlingL---'I have found that a hand set may be constructed in such a way that'the um-d'irectional mutual impedance iskept within an: allowable rangeeven when highly efficient-transmitters and receivers are used. 1 .In' general, the impedances of the receiver a'ndtransmitter are determined'by the-char- .acteristics of the electrical circuits with which they are-to be employedfandthese for the purpose of the present description.

The mechanical'construction of the hand 9 set directly affects the acoustic and mechani cal coupling between the receiver and trans--' mitter. The first-is due-to the agitation of the transmitter and 'diaphragm' -rcaused by the sound waves.- emitted by I the receiver when variable current passes through it,- and the other isdue to th'ecmechanicalvibration of the receiver case transmitting vibrations through the handle to the transmitter. Due. to the relative-positions of the receiver and transmitter in. a hand setthe acoustic coupling is usually of too small a value to be of serious conse uence. For this reasonit is not considere herein. 1 a.

Themechanical coupling through the handle 'is best expressed quantitatively as the ratio of the displacement ofthe transmitter end of the handle to the force at'the receiverendproducing it. Like the transfer admittance in an electrical circuit the mechanical coupling forsinusoidal. forces is a complex operator possessing both magnitude and phase. 1

I have found that high mechanical cou-- pling results from the mechanicalreson'ance of thehand set orapart thereof. The reso- '600 and 1800 cycles per second, which is roughly the most important range of voice frequencies. In order to make the mechanical coupling low, it is necessary to prevent, therefore, as far as practicable, all mechanicalBresonances within such frequency range.

manner that all its mechanical resonaces occur below 600 cycles, the mechanical couling might be reduced to a-minimum value. his would necessitate the use of relatively heavy and flexible parts, and would produce a hand set which could not be readily adapted for use in ordinary telephone substations. Moreover, the handle of such a hand set would possess a series of overtones which would fall within the frequency range stated above. n 7

It is preferred. therefore, to make the parts of the hand set light and rigid, to mal ce the walls of the cases for the transmitter and receiver rigid, to make all of the joints between the various parts extremely tight to raise their elasticity, to so construct the handle that its fundamental frequency of vibration is as high as compatible with the conditions imposed upon it by its use, and in this manner produce a hand set possessin a low mechanical coupling.

. he vibration of such a handle under conditions of operation takes place in transverse waves, and the frequency of its fundamental mode of vibrationor lowest frequency for standing waves, is approximately propor tional to its radius of gyration in the direc tion of wave displacements and to the velocity of sound through the medium, the velocity of .sound. being proportional to the square root of the ratio of the volume modulus of elasticity of the material to its density. In order to raise the fundamental frequency of the handle, it is necessary to em ploy a cross section having a large radius of gyration as stated above, or a material with'a high ratio of modulus of elasticity to density or a combination of the two. I have found that the fundamental mode of vibration of the handle should not be lower than constructing the hand set in such a.

be found useful in'designing cases having a definite frequency characteristic. The component parts of the cases also must be held tightly together. Moreover, wherever two flat surfaces are in contact, the surfaces must be accurately made and the screws or other means employed for holding the parts together must be tight. In some cases it. is desirable to employ a washer of paper or other yielding material between the parts to take up any unevenness in the surfaces.

The general expression for the frequency ofabove.

(3) The material of which the handle is made.

A suitable hand set may be formed by applying the well-known theory of vibrating bars, the general expression forwhich is where n is the frequency of the gravest mode of transverse vibrations, A a constant including the length which is essentially fixed by the head dimensions of the user, K the radius .of gyration, E the modulus of-elasticity and P the density. By the application of this formula, the handle may be calculated for any uniform structure which may afterward be modifiedto suit the artistic sense providing the radius of gyration is not reduced.

The radius of gyration as expressed above, should be as large as the construction of the hand set will allow. By increasing the cross section in the direction indicated in (2), the mechanical coupling is reduced since this will increase the mass, elasticity and internal resistance of the handle.' The selection of materialfor the hand set handle will de pend upon the ratio of the modulus of elasticity tothe density. of material, as well as its internal damping or molecular friction under flexural strain. If the ratio of the modulus of elasticity to density be alone considered, it has been found that metals, such as aluminum or steel or their alloys are best suited as a handle for the hand set. There lit are, however, other materials, such as phenol condensation products which possess; a

higher internal damping than metallicsubstances and for this reason are to be;.p referred in many cases.

of the transmitter and receiver. Referring to the drawings, Flgs-lv an vquirements as set' 'fort-h 'hereinabove for the production of a hand set which will give a maximum efiiciency ofoperation without howling. Such a construction, however, is not a desirable one since the handle would be diificult to grasp and since it is not orna: mental. The essential features of such a construction, however, may be embodied in a form such as that shown in Figs. 3 and 4 which is suitable for use. In Fig. 3 the handle is shown as being made in one piece with the receiver and transmitter cases. Such a construction. while being the best for use, is somewhat difiicult to manufacture. For this reason it has been found more desirable to make the handle and instrument cases separately and attach them together in the manner as shown in Fig. 5. In this 'figure, the receiver case is fastened to the handle 11 by means of the screws 10, the paper washer 9 being inserted therebetween. This washer serves to compensate. for any unevenness in the surfaces of the case and handle. Any other manner producing rigid clamping between the instrument cases and the handle may be used.

By proceeding in accordance with the considerations hereinabove stated, I have been able to having:

1. A handle, which with the receiver and transmitter attached, has but one resonance to transverse waves below. a frequency of 1800 cycles per second.

2. Other parts and joints which do not resonate below 1800 cycles per second.

The maximum uni-directional mutual impedance of such a set occurs at a frequency higher than 1800 cycles per second.

In order to connect'the transmitter and receiver electrically, it is foundconvenient when a non-conducting material is used for the handle to imbed the electrical conductors therein. It is also desirable to imbed The same considerations} apply with equal, force to the material to be used fo'r the case's .32. illustrate ahand set having a handle beam construction; Figs. 3 and 4. re resent, the preferred construction fora han setto.

accomplish the objects of the present inven-'v '.2;JAY unitary istructure tached thereto,

produce a telephone hand set cure the instrument ;;cases.-.' thereto. handle having such inserts, howeve sidered to be {a .onepiece l handle-3p that it is not longitudinally :dicbnt lions between the instrument H lVhatis'claimd-is; I

1. A unita tructure. ,.comprising a I transm tter, a, receiver and a; support there tienal .Inutual h h r. Ethan '80 es-pr s ondcomprising transm ttingjand receiving port therefor, said su quency of fundamental exceeding approx mately ,jey'cles per second. f p

. 3. A. unitarygstruct mitting andre'ceiving was embers, a

t ansagsup port therefor, "said support 'fhaving'ibutone l resonance to transverse waves jbelo wyaf frequen X of 1800 cycles per second. unitary structure compri a highly eflicient transmitter and a-ree'eiver, and a handle with a. frequency of fundamental mode of vibration-{exceeding approximately 1,000 cycles per second.-

5. .A telephone hand setcomprising. a handle with a frequency of fundamental mode 'of vibration exceeding approximately 1000-cycles per second of said hand-set being of high elasticity and resonating not'below 1800 cycles per second.

6. A telephone handset comprising a handle, receiver and transmitter cases 'atsaid receiver and transmitter .cases having no resonance below 1800 cycles per second. w l

7. A telephone hand set comprising a handle, receiver and transmitter cases attached thereto, the joints between said handle and said cases having no resonance below 1800 cycles per'second.

8. A telephone; hand set comprising a handle in one piece, having a frequency of fundamental mode of vibration exceeding apprtxzimately 1,000 cycles per second.

9. handle in one piece, receiver and trans mitter cases attached thereto, said receiver and all of the joints.-

telephone hand set comprising a suitable metal parts in a handle'to se. 65.

con

"dedi.

and transmitter cases having no resonance below 1800 cycles per second.

.10. A telephone hand set comprising a handle having means imbedded therein to. secure instrument cases thereto, said handle having but one resonance to transverse but one '12. A telephone handset comprising a handle composed of phenol condensation product, said handle having but one resonance to transverse waves below a frequency of 1800 cycles per second.

13. A telephone hand set comprising a receiver, a. transmitter, a handle having a second.

14. A telephone hand set comprising a handle, a receiver case, and a transmitter case attached thereto, said receiver and transmitter cases being formed of a' highly elastic material of such thickness that the fimdamental mode of vibration exceeds 1800 cycles per second.

15. A telephone hand set comprising a transmitter, a receiver, and a common supporting means, said means com rising an.

gyration elastic body having a radius 0 such that its fundamental mode of transverse vibration exceeds approximately 1000 cycles per second.

16. A telephone hand set comprising a transmitter, a receiver, and common supporting means, said. means comprising an elastic body having a radius of gyration such vthat it has but one resonance to transverse waves below a frequency of 1800 cycles per second.

In witness whereof, I hereunto subscribe my name this 22nd day of December A. D.,

ALFRED S. CURTIS. 

